Method and apparatus for explosively deforming cylinders

ABSTRACT

A METHOD OF AND APPARATUS FOR EXPLOSIVELY COLD DEFORMING A METALLIC HOLLOW CYCLINDRICAL WORKPIECE. A SUBSTANTIALLY CYLINDRICAL EXPLOSIVE CHARGE HOLDER HAVING A RING SHAPED CHARGE ABOUT ITS OUTER PERIPHERY IS MOUNTED COAXIALLY WITHIN THE HOLLOW CYLINDRICAL WORKPIECE. THEN THE HOLDER, CHARGE AND WORKPIECE ARE SUBMERGED IN A VESSEL FILLED WITH FLUID ENERGY TRANSMITTING MEDIUM. THE CHARGE IS DETONATED AND THE EXPLOSIVE ENERGY IS TRANSMITTED THROUGH THE MEDIUM TO EXPAND THE WORKPIECE OUTWARDLY PREFERABLY THE CHARGE HOLDER HAS A CIRCUMFERENTIAL CONCAVE RECESS WITHIN WHICH THE CHARGE IS MOUNTED. IT IS ALSO POSSIBLE TO EXPLOSIVELY DEFORM HOLLOW CYLINDRICAL WORKPIECES OF RELATIVELY GREAT HEIGHT BY FIRST EXPLOSIVELY DEFORMING THE BOTTOM PORTION OF THE WORKPIECE AS DESCRIBED ABOVE, INVERTING THE PARTIALLY DEFORMED WORKPIECER, AND DETONATING A SECOND CHARGE TO COMPLETE THE DEFORMATION.

Jan. 26,

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ATTORNEYS United States Patent 3,557,589 METHOD AND APPARATUS FOR EXPLOSIVELY V DEFORMIN G CYLINDERS Ludwig Schwarz, Kapfenberg, Austria, assignor to Gebr. Bohler & Co., Aktiengesellschaft, Vienna, Austria Filed July 25, 1968, Ser. No. 747,639 Int. Cl. B21d 26/08 U.S. C]. 7256 Claims ABSTRACT OF THE DISCLOSURE Preferably the charge holder has a circumferential con-- cave recess within which the charge is mounted. It is also possible to explosively deform hollow cylindrical workpieces of relatively great height by first explosively deforming the bottom portion of the workpiece as described above, inverting the partially deformed workpiece, and detonating a second charge to complete the deformation.

Methods for the cold working of metallic workpieces by means of explosive pressure have become more and more popular in more recent years. The prior art comprises principally two methods which are called in the literature standoff operations and contact operations.

In the standoff operations a transmission medium is used for the pressure wave formed, all liquids being on the whole suitable, but in most cases, and this is for economical reasons alone, water is used. In the contact operations the workpiece to be deformed is brought in direct contact with the explosive. A number of publications deal with the deformation of sheet metal plates, which are pressed in a deep-drawing process into dies or molds. The expansion or impression of hollow cylindrical bodies is likewise mentioned in the literature, and with but a few exceptions they are concerned with thinwalled tubes. The literature always speaks of a centrally arranged charge, and the limits of expansion is usually determined by dies.

The expansion of ring caps by means of explosive pressure has likewise been carried out. In this case too, the explosive is arranged in the center of the ring cap. Water is used as the transmission medium. Dies are not used, in this case, to limit the expansion, but in this known process the ring cap is formed by a subsequent straightening process, or a considerable machining tolerance must be allowed.

A method is also known where an axially symmetrical charge holder is used, on whose surface the charge is so arranged that the amount of charge varies constantly in the longitudinal direction, the charge surface assuming approximately the form of a hyperboloid of revolution. The object of this arrangement is to equalize the explosive pressure in order to avoid any bulging of the hollow cylinder to be deformed about halfway of its height. But it was found that just the opposite effect is achieved by this process than was intended, since shock wave concentrations are formed on the concave side of the charge in curved charges according to todays generally accepted findings. In the extreme case, the result is the well-known hollow charge effect. The hollow cylinder is even more irregularly deformed than would be the case, for example, if a simple rod charge were used.

The object of the present invention is a process for the cold expansion of metallic hollow cylinders which avoids the above-mentioned disadvantages. The expansion process is carried out by using water as an energy transmitter and takes place as follows:

The hollow cylinder to be expanded rests on a supporting plate or on an anvil. Into the hollow cylinder is introduced an axially symmetrical, preferably cylindrical insert, hereafter called the charge-holder, on the peripheral surface adjacent the cylinder is placed the explosive which is shaped as a ring preferably as a circular ring with a circular cross section, since the convex curvature of such a charge leads to an effect which is the opposite of the undesired hollow charge effect. Rings with an oval cross section can also be used with advantage for the purpose of the invention, :but rings with other, for example, rectangular cross section, are also within the scope of the invention, provided the boundary surface of the charge with the energy transmitting medium is not concave. In the preferred embodiment, at least one concave recess is provided in the surface of the said chargeholder, into which the explosive is placed. The chargeholder is preferably made of metal, particularly steel. The expansion process can be so effected that the entire arrangement described above is submerged in water where the explosion takes place, but a water filling can also be arranged solely between the charge-holder and the hollow cylinder workpiece. The explosion thus takes place in the water between the two metallic bodies, namely the chargeholder and the hollow cylinder to *be expanded. The charge-holder has the function of a reflector and allows a more accurate control of the explosion than would be possible by using a molded central explosive charge alone. This charge-holder insert has three functions in the explosion:

(1) The force components of the detonation acting in vertical direction are absorbed by the form stability of the insert. This way an undesired scatter effect is prevented to a great extent.

(2) The force component of the detonation directed radially toward the center is reflected to a major part .by the insert, and is therefore utilized. Due to this improved utilization of the energy released in the detonation, only about half the amount of explosives is required for the same expansion as is required in conventional methods.

(3) Due to the above described arrangement of the explosive it is possible to expand the hollow cylinder workpiece so that a larger diameter hollow cylinder is obtained again after the expansion with such minor deviations from the desired form that it falls within the machining tolerance.

In carrying out the expansion it is advisable to provide a sufiicient height of water above the explosive charge so that the detonation is completed before the cavity formed in the water by the detonation has traveled to the water surface. It was found in tests that the water damming is sufficiently effective when the distance of the water level from the horizontal plane of symmetry of the ring-shaped charge is twice the charge diameter. By charge diameter is to be understood not the inside diameter of the ring-shaped charge, but the mean diameter as taken from the vertical planes of symmetry of the charge.

In order to avoid a general bulging in the center during the expansion of higher hollow cylinder workpieces, it is advisable to effect the expansion in two stages, a hollow truncated cone being formed in the first state. A ringshaped explosive charge is detonated in the proximity of the supporting surface, this charge being arranged not higher than at about one quarter of the height of the hollow cylinder to be expanded. The reference level is the horizontal plane of symmetry of the explosive charge. The hollow truncated cone is formed due to the hollow cylinder workpiece expanding at its base. In the second stage, the hollow cone workpiece thus produced is placed with the truncated end at the bottom, so that the smaller diametered end now becomes the base. If the expansion process is repeated in the above described manner, an enlarged hollow cylinder is formed again from the hollow truncated cone.

FIGS. 1, 2 and 3 show diagrammatical arrangements for carrying out the method according to the invention.

FIG. 1 shows the charge-holder 1 with the ring-shaped charge 2, which preferably does not directly touch the charge-holder 1. Hollow cylinder workpiece 3 to be expanded is arranged concentric of the charge 2, and the whole assembly is arranged in a basin 5 filled with water 4.

FIG. 2 shows a different embodiment charge-holder 1 which has a concave recess for receiving ring shaped explosive charge 2.

For carrying out a two-stage method it is not necessary that the charge-holder be as high or higher than the hollow cylinder workpiece 3 to be deformed. An insert chargeholder 1, as represented in FIG. 3, likewise leads to the same result but in a two step process. In the deformation of larger rings, as they are used in practice, the smaller charge-holder insert will be used to save costs. In the deformation of smaller rings, it is advisable in the interest of easier handling to use charge-holder insert protruding beyond the explosive arrangement.

The subject of the present invention is therefore an arrangement for the explosive deformation of metallic hollow cylinders by means of explosive charges arranged inside the hollow cylinders, using water as an energy transmitter, and the invention consists in that the explosive is arranged on the surface of a cylindrical chargeholder in the form of a ring. This charge-holder together with the charge is introduced into the interior of the hollow cylinder workpiece and the charge is detonated between the charge-holder and the surrounding hollow cylinder workpiece. This method is particularly suitable for expanding cap rings.

The form of the shock wave and its effect on the hollow cylinder workpiece to be expanded can naturally be varied in many ways, for example by selecting a suitable width and depth of the concave circumferential recess in the charge-holder or by the shape of the charge-holder. It is also possible to design a concave circumferential recess so that its width and/or depth varies regularly or irregularly over the circumference of the charge holder, and the recess can have interruptions in the extreme case. If relatively large charges are arranged on relatively small charge holders, it is advisable to prevent any direct contact between the charge and the charge-holder, for example by inserting spacers, otherwise there is a possibility that the charge-holder will be destroyed by the so called contact-operation.

EXAMPLE A cap ring of nonmagnetic steel with an inside diameter of 380 mm., an outside diameter of 520 mm., and a height of 450 mm., was to be expanded by 15% related to the mean diameter.

A ring shaped explosive charge with circular crosssection was inserted between the hollow cylinder workpiece and the charge-holder so that it touched neither the hollow cylinder workpiece nor the charge-holder and was spaced 70 mm. from the bottom of the workpiece. The weight and the thickness of the charge depends upon the characteristics of the type of explosive used. The charge for the desired expansion was 2000 g. gelatin-donarit. The depth of water above the explosive charge was 380 mm. Due to the detonation of the explosive charge, the

hollow cylinder workpiece was deformed into a hollow truncated cone, which showed the desired expansion of the bottom portion adjacent the charge. The top portion remained practically unchanged.

Then the hollow truncated cone workpiece was inverted with the unchanged portion at the bottom. In this situation the same quantity explosive charge would cause a greater deformation effect than the first charge. This would be due to the increased damming effect of the greater water volume. The charge must therefore be reduced by about 7% in order to produce a properly expanded hollow cylinder.

The method according to the invention has led in practice to great precision and simplification, for example, in the cold expansion of cap rings, and to considerable savings in explosives.

What I claim is:

1. A method of explosively cold deforming a metallic hollow cylindrical workpiece in the absence of external dies comprising the steps of assembling said workpiece, a substantially cylindrical charge-holder and a ring shaped charge so that said charge is interspaced between said holder and said workpiece, the boundary surface of said charge positioned toward said workpiece being convex or flat; submerging said assembly in a fluid explosive-energy transmitting medium; and detonating said charge so that the explosive energy transmitted through said medium causes said workpiece to expand outwardly.

2. A method as claimed in claim 1 wherein spacing elements are used to space said charge from said holder so direct contact is prevented; and said holder is a smooth cylinder.

3. A method as claimed in claim 1 wherein said holder has a circumferential concave recess juxtaposed to which said charge is located.

4. A method as claimed in claim 1 wherein said fluid medium is water and the surface of said water is spaced at a distance twice the diameter of said ring-shaped charge above the horizontal plane of symmetry of said charge.

5. A method as claimed in claim 1 wherein said charge is arranged at a height which is not more than one quarter the height of said workpiece; the first detonation forms a truncated conelike first shape in said workpiece which is then reassembled so that the smaller diameter portion is juxtaposed to a second charge which when detonated forms an expanded substantially cylindrical final shaped workpiece.

6. Explosive deforming apparatus for explosively cold deforming a metallic hollow cylindrical workpiece in the absence of external dies comprising a substantially cylindrical charge-holder placed within a ring shaped charge of large diameter, both of which being adapted to be inserted within said workpiece to form an assembly, said charge having a boundary surface positioned toward said workpiece which is convex or flat, and a vessel for containing a sufficient depth of fluid explosive energy transmitting medium to submerge said assembly so that when eaid charge is detonated the explosive energy is transmitted through said medium to cause said workpiece to expand outwardly.

7. An apparatus as claimed in claim 6 wherein spacing elements are located on said holder and adapted to space said charge from said holder so direct contact therebetween is prevented; and said holder is a smooth cylinder.

8. An appaartus as claimed in claim 6 wherein said holder has a circumferential concave recess juxtaposed to which said charge is located.

9. An apparatus as claimed in claim 6 wherein said medium is water and its depth is such that the distance of the water level from the horizontal plane of symmetry of the ring-shaped charge is twice the charge diameter.

10. An apparatus as claimed in claim 8 wherein the charge is arranged at a height which is not more than one quarter the height of said workpiece; and the longitudinal 5 6 height of said recess is greater than the longitudinal height 3,167,122 1/ 1965- Lang 7256 of said charge. 3,175,618 3/1965 Lang et a1. 7256 References Cited 3,206,845 9/1965 Crump 7256 UNITED 3,344,509 10/1967 Kunsagi et a1. 7256 2,779,279 1/1957 Maiwurm 29-421 5 RICHARD J. HERBST, Primary Examiner 3,156,973 11/1964 Lieberman et a1. 7256 

